A closed ride control system for vehicles of the type mentioned in the introduction is described in the European patent application EP 1 243 447 A2, which is incorporated by reference. The closed ride control device known from that document comprises two or four controllable directional valves, a pressure medium reservoir and at least two pressure medium chambers, each having a controllable directional valve. With this ride control system, pressure medium can be transferred from the pressure medium chambers to the pressure medium reservoir and vice versa. Furthermore, air from the atmosphere can be conducted into the pressure medium reservoir, the compressed air being dried in an air dryer before entering the pressure medium reservoir. In order to regenerate the air dryer, pressure medium from the pressure medium reservoir can be decompressed via a throttle and conducted to the atmosphere through the air dryer via a discharge valve, in the opposite direction in comparison to the air-drying. The regeneration of the dryer in the opposite direction is disadvantageous because it is less effective and is therefore associated with a relatively high consumption of air. Furthermore, the system contains line paths which in certain operating states remain filled with moist air, which can be removed only subsequently by additional regeneration cycles.
According to the closed ride control system of DE 102 31 251 C1, which is incorporated by reference, direct filling of the air springs, which may occur, for example, after a long stationary period of the vehicle, is possible, since the compressor can draw air directly from the atmosphere via an additional line leading to its inlet, which air can be conducted to the air springs directly from the outlet of the compressor. In this case, however, the direct filling of the air springs is carried out with moist air which must subsequently be dried by means of suitable regeneration cycles.
The regeneration cycles required for the removal of the air humidity involve additional compressor running times, since the air must first be conveyed into the system and then discharged, which must be taken into account with regard to the total service life of the compressor. These additional compressor running times entail a reduction in the availability for use of the ride control system itself, since the compressor and valves are in use longer and for thermal self-protection must periodically be left unused. Additional drying of the system air during operation of the ride control system is possible only as the air is conveyed from the air springs to the accumulator. During the conveying of air from the accumulator to the air springs, the air is diverted to the air springs upstream of the dryer.
An air suspension system for a vehicle, in which air always flows through the dryer as it is conveyed by a compressor, and in which both the air drying and the regeneration of the dryer take place in the same direction of flow, is known from DE 102 40 357 A1, which is incorporated by reference. The structure of this system is complicated, since a compressed-air controlled 4/3-way valve, which can be moved to three switching positions as a function of the air pressure in the system, is arranged upstream of the dryer, the first of which switching positions opens a passage of relatively large cross section through the 4/3-way valve, while the second switching position opens a throttled passage cross section and the third switching position releases a throttled discharge to the atmosphere. A regeneration of the dryer takes place in this third switching position. The compressed air for switching the 4/3-way valve is branched off upstream of the dryer and therefore contains moisture. This air cannot be dried by a system function since it is located in a part of the line without through-flow, entailing a risk of freezing-up of the 4/3-way valve, which also serves as a discharge valve.